Electrical recorder



April l1, 1939. o. BRUNNERT ET A| ELECTRICAL RECORDER Filed Oct. 29, 1937 l g f.v

Patented Apr. 11, 1939 UNITED STATES PATENT OFFICE ELECTRICAL RECORDER Application October 29, 1937, Serial No. 171,602

In Germany November 2, 1936 7 Claims.r

This invention relates to improvements in electrical records, and more particularly to improvements in electromagnetic recorders of the type used in conjunction with power amplifying circuits in making sound records. One object of the invention is the provision of a device of this character which is simple, durable, and practical in construction, and is able to record the Wide band of frequencies extending from 40 to 1d 10,000 cycles without distortion and with a good efficiency. n

In the latest development of amplifiers and pick-ups, the band of frequencies which are reproduced without distortion has been consider- 15 ably extended. Nowadays, there are no difliculties in reproducing the band from 40 to 10,000 cycles free from distortion. yThis requires improving the method and devices for making talking machine records.

The known magnetic recorders comprising a reed armature which is surrounded by the voice coil and fixed at one end, while at the movable end lying between the two pole pieces the recording stylus is mounted, are far betterthan 25 other constructions of recorders. Such known system is shown in Fig. 4. Between the poles of the U-shaped lpermanent or coil excited magnet m there arelocated the pole pieces p. Between the pole pieces extends the movable end of the reed armature b, which carries the cutting stylus st. The other end of the reed b is clamped, soldered, or welded to the non-magnetic bridge e. The coil surrounding the reed b is energized by the output current of the power 35 amplifier and causes the reed to oscillate according to the audible frequencies to be recorded.

These systems are easier to design and better in quality. The design is very simple and the adljustment of the reed armature between the pole k40 pieces is not difficult. The band of frequencies extends nearer to the limit of audible frequencies. When the coil of the recorder is supplied with the same electrical energy the mechanical amplitude of the stylus is greater than with other recorder systems. It is possible to obtain amplitudes of .06 mm. with such reed armature recorders having a resonance frequency of 5,000- 6,000 cycles. Since the resonance frequency limits the band of frequencies to be recorded, 5,000- 6,000 cycles is the upper limit of the audible frequencies which can be recorded.

If such a system should be so constructed that its resonance frequency lies at the upper limit of the audible frequencies and that the same k5:5 amplitude of .06 mm. could be recorded, the dimensions of the reed armature and the stylus would be such that the parts are very fragile and manufacturing is practically impossible.

If the dimensions of the reed armature, thickness or width, or both, are enlarged in order to raise the resonance frequency, the amplitude of the reed decreases `considerably when the coil is energized with the same current, because although the magnetic deiiecting force increases proportionally with the dimensions of the reed, the stiffness of the reed increases with the third power of the thickness and increases proportionally to the width. Furthermore, since the magnetic deiiecting force is lowered, the damping of the amplitude of the oscillating reed by the material of the disc to be cut (e. g., wax) becomes more important. The other way to raise the resonance frequency, namely, by decreasing the dimensions of the reed, is impracticable because the strength of the reed and of the stylus becomes too small and manufacturing too difficult, as previously stated.

The problem to be solved, accordingly, was to maintain the relation between magnetic deflectng force and stiffness, and to lower the oscillating mass of the reed without lowering the magnetically effective cross-section of the reed. Under such .conditions the maximum amplitude of the reed must be the same.

According to the invention, this problem is solved by dividing the reed armature into an oscillating part and one or more fixed parts. Preferably the oscillating part is formed in the shape of the letter U, E, or T. The selected prole of the armature is in any case preferably completed to a rectangular shape by the fixed parts. It is evident that other forms, e. g., a trapezoid form, also may be composed of the fixed and the oscillating parts of the reed armature. 'I'he distance between the armature parts can be made very small and, therefore, the alternating flux through the armature is substantially equivalent to the flux through a homogeneous rectangular armature. In practice this distance amounts to from .01 mm. to .1 mm. 'Ihe thickness of the oscillating part lof the armature may be made equal to the thickness of the fixed parts, because the amplitudes of the oscillating part are small (.06 mm. in maximum) as compared to the thickness of the armature (.3 mm.). cial constructions, nevertheless, it may be advantageous to make the fixed parts thicker than the oscillating parts in order to avoid that the oscillating part moves outside the boundary 4planes of the xed partsx In spe- The mass or the moment of inertia of the mass is thereby considerably lowered, without varying in practice the magnetic properties. Tests have proved that With such a reed armature system having a resonance frequency of more than 10,000 cycles amplitudes of .08 mm. were obtainable, which could not be obtained when using the known recorders. The new recorder is adapted to cut laterally undulating grooves as well as hill and dale grooves; the best results, however, are obtained in recording laterally undulating grooves.

In order that the invention may be better understood, attention is directed to the accompanying drawing forming part of this specication, and in which- Fig. 1 is a specific embodiment of the reed armature according to the invention;

Fig. 2 is another embodiment of the reed armature;

Fig. 3 is a third embodiment of the reed armature; and

Fig. 4 shows the above described known reed armature recorder in section, improvements in which are the subject-matter of this invention.

In Figs. l to 3, the oscillating part of the reed armature is shown at b, and has mounted on it the stylus a. The letter c indicates the xed part or parts of the reed armature. The voice coil surrounding the reed armature is not shown.

'The lower end of the reed armature opposite to the stylus a. is xed to the bridge c, Fig. 4. In Fig. 1 the oscillating part is T-shaped; in Fig. 2 it is U-shaped; and in Fig. 3 it is E'shaped. The oscillating part b and the xed parts c are separated by the distance d. It is to be noted that the E-prolle according to Fig. 3 is particularly useful on .account of its stiilness.

The fixed and oscillating parts of the armature cooperate in forming a rectangular prole. The distance d between the oscillating and the xed parts may amount to from .01 mm. to .1 mm. without appreciably reducing the magnetic properties of the recorder.

It will be apparent to those skilled in the art that this invention is susceptible of many variations, and its scope therefore is not to be limited by the particular details given in the above description and drawing, but it is to be defined en tirely by the appended claims.

What we claim as new and desire to secure by Letters Patent is:

1. In electromagnetic recorder, a magnetic circuit including a magnet and an air gap between the poles thereof, a voice coil, a core inside said coil comprising fixed and movable parts, both of magnetic material and the movable part extending into said air gap, and a stylus secured to said movable part.

2. In an electromagnetic recorder, a magnetic circuit including a magnet and an air gap between the poles thereof, a voice coil, a movable armature extending through said coil and into said air gap, the portion of said armature which passes through said coil being of reduced crosssection, one or more xed pieces of magnetic material within said coil and adjacent said armature but out of contact therewith, and a stylus secured to said armature.

3. In an electromagnetic recorder, a magnetic circuit including a magnet and an air gap between the poles thereof, a voice coil, a reed armature extending through said coil and into said air gap, said armature having a section cut away where it passes through said coil, a fixed strip of magnetic material inside said coil and substantially lling the space in said armature where said section has been cut away, and a stylus secured to said armature.

4.' In an electromagnetic recorder, a magnet having pole pieces, a reed structure comprising fixed and oscillating parts in the same plane, a non-magnetic bridge supporting said parts with the oscillating part extending between said pole pieces, a voice coil surrounding both the xed and oscillating parts, and a stylus secured to the oscillating part.

5. In an electromagnetic recorder, a magnet having pole pieces, a reed structure comprising an oscillating part in the shape of the letter T, U, or E, and one or more xed parts in the same plane and of the required shape to give said structure a generally rectangular outline, means supporting said structure with the oscillating part thereof between said pole pieces, a voice coil surrounding both the ixed and oscillating parts of said structure, and a stylus secured to said oscillating part. n

6. In an electromagnetic recorder, a generally U-shaped magnet having pole pieces spaced apart to form a restricted air gap, a bridge of nonmagnetic material extending between the legs of said magnet, a voice coil between said bridge and pole pieces, a reed armature comprising a portion located in said air gap and one or more narrow strips secured to said bridge and extending through said coil to the said portion located in the air gap, one or more separate strips of magnetic material secured to said bridge and extending within said coil in the same plane with said iirst mentioned strips and in interspersed relation therewith, and a stylus attached to the said armature.

'7. An electromagnetic recorder as dened in claim 3, in which the thickness of the fixed strip is at least as great as the thickness of the reed armature.

EDWIN SEVERIN. OTTO BRUNNERT. 

